1. Field of the Invention
The present invention relates to an automatic sound volume correction method and apparatus for use in audio equipment for vehicles, and more particularly, to an automatic sound volume correction method in which the volume correcting value thereof is inferred by fuzzy (logic) control.
2. Description of the Related Art
Unlike the inside of a house, the inside of a vehicle is full of noise, such as engine or air sound. The level of such noise changes with the running speed or location. Thus, when the driver or passenger listens to a radio broadcast or the reproduced sound of a cassette tape or a compact disk, he or she sets the volume according to the noise level. That is, when the noise level is high, he turns up the volume as compared to when the vehicle is running in a quiet state. As mentioned above, the noise level changes with the running speed or location. Therefore, the driver must adjust the volume according to the noise level, which is troublesome and undesirable from the viewpoint of safe driving.
Accordingly, it has been proposed to automatically correct the volume using a detected low-frequency noise level. FIG. 8 shows the structure of such a conventional automatic sound volume system. Reference numeral 1 denotes a microphone for detecting sound waves in a vehicle; 2 denotes an amplifier for amplifying the detected sound signal; 3 denotes a low-pass filter for detecting a low frequency band component at 50 Hz or below contained in the detected sound; 4 denotes a rectifying circuit including resistor R and capacitor C; 5 denotes an automatic volume control portion which is a microcomputer; and 6 denotes audio equipment. The automatic volume control portion 5 determines a volume correcting quantity according to the level of a low-frequency band component, and inputs the determined value to the audio circuit 6 to control an electronic volume (not shown). That is, when the level of the low frequency band component is high, it is determined that the noise level is high and then the volume correcting quantity is increased to turn up the volume. When the level of the low frequency band component is low, it is determined that the noise level is low and the volume correcting quantity is reduced.
Generally, a high level low-frequency band component signifies a high noise level. However, this is not always true: the level may change due to vibrations of the vehicle or the like. Also, the frequency band which is noisy and offensive to the ears of a human being is not a low frequency band of 50 Hz or below; a noise having an intermediate frequency band level is the most offensive.
For the above-described reasons, it is impossible to perform volume correction suitably by the conventional method in which volume correction is performed using a single condition, i.e., using the low frequency band component alone. As a result, the listener may undesirably experience an odd listening sensation.